At her very lowest ebb, Marian Keyes hated everyone who loved her. She hated their support, hated their help, hated that their existence meant that she couldn't kill herself.

"I know this sounds really selfish, but I resented them," says Marian in the front room of her house, a home so full of colour that it seems to scream joie de vivre. "I wanted to kill myself and I couldn't, because of them. At one point, my sister Caitriona rang from New York and said, 'If you kill yourself, me and Mam will never, ever get over it.

"I didn't really believe her, because I really thought that I was nothing but negative, and if I was gone, then no one would bring them unhappiness. That was a legitimate belief at the time," Marian says, echoing the experience of a lot of people who have considered suicide and later found a way to get true perspective on those false thoughts. "But good for Caitriona. She gave it to me straight."

Marian Keyes, though she is extremely careful in how she characterises it, and guarded when she needs to be, gives it very straight on her depression. Further, she is helpfully honest about how its arrival blindsided her.

"I was shocked by it," Marian says. "I really was. Because you have this image of a depressed person as sitting beside a window, quietly, looking out at the rain, as a little tear trickles down their face and that's not what it was.

"It wasn't like that for me; I was crazed with fear," she explains. "And it was relentless. And I suppose when I wrote the last book [The Mystery of Mercy Close, in which the heroine is depressed and suicidal], I wanted to write about how it really is. And most people haven't a notion. I hadn't a notion, you know? I would have happily described myself as melancholic, and I am, but I know now that that was different."

For a bit, we talk about how it is the human condition to go around thinking that bad things happen to other people and feel outraged and aghast when they then happen to us.

And Marian, having dealt with alcoholism in her late 20s and, in her 30s, having come to terms with not having children, thought she'd had her knocks. "And I was doing all the right things," she says. "I won't go into it too much, but I follow the 12-steps programme, and I was so in touch with things like my shame and not burying it. I've worked on myself. Oh, God, I hate that phrase, but I've stayed connected to the things that distress me.

"So I thought I was fine. I thought I wasn't one of those people who keep things bottled up and then there's an explosion. But I was still knocked."

Marian describes her condition, at its worst, as constant terror that reduced her to a childlike state. She pours out pity for her husband, Tony, who coaxed her through every day, who supported and surrounded her with a sense of security, even when she couldn't sense it.

"Every single day was a waking nightmare," she says, "and I'm choosing my words very carefully. It was like being awake in a nightmare. I couldn't sleep, but when I did sleep, I'd wake up in terror. The biggest feeling was extreme fear. And I tried so many things to make myself well, but it went on for so long."

Eventually, Marian wrote a short note on her website, explaining that she couldn't eat, sleep, function, and the response was huge, both positive and negative. Then, in 2012, she published Saved By Baking, a cook book that gave an account of how cake-making had helped her through the darkness; The Mystery of Mercy Close, also published that year, featured a detective with depression. Writing helped, she admits, even if it was hard to do it sometimes. She also discovered Twitter in that period, to her delight, because it helped her to feel sociable, if only virtually, and still part of the world outside her home.

Her latest, 12th novel, The Woman Who Stole My Life, is the book of which Marian Keyes says she is most proud. She loves the central character, Stella Sweeney, and her positivity, but she also loves it because the writing of it coincided with a return to herself.

Even today, as she talks and laughs and seems every bit her usual effervescent self, Marian would not call herself 100 per cent free of the depression that began over five years ago. "The fear isn't too bad now," she says. "I feel normal so much of the time, and I feel that when the fear does happen, that I can be compassionate with myself about it."

As well as being compassionate, Marian knows she has to be careful with herself. So, she is tiptoeing back into the world, quietly promoting her new book, The Woman Who Stole My Life, and preparing herself for a few public readings.

It's tiny steps, and if it all feels too much, she'll draw back again. So proud is she of the new book, though, that Marian wants to trumpet it. She's proud to have written what is a very hopeful, life-affirming book during a time when she felt so low, and she's proud to have been able to write at all.

The Woman Who Stole My Life tells of Stella Sweeney, a relatively ordinary woman aged 41 and one quarter, whom we meet as she flounders after an extraordinary few years of near-fatal illness, massive literary success with the self-help book that followed her illness and then, horribly, a crash back to reality.

We go with Stella as she retraces the arc of the recent past, trying to make sense of the best and worst of times, a big love affair, a huge betrayal and, in parallel to all the exciting highs and lows, a lot of the mundane but amusing ordinary stuff of being a woman and a mother and a flawed human being.

Marian Keyes loves Stella, who is older than the characters we are more used to her writing about. "I think there is nothing that has given a voice to the vulnerability of women in their late 30s and their 40s," Marian says, "We're more experienced and in many ways we're wiser, but we're still vulnerable to pain. In some ways, life gets easier, you're better at saying no and you know your limitations, but you've feelings; you still love and you still lose."

And, Marian is very keen to emphasise, you still have a sex life. And, boy, does she give Stella a sex life. "I've been quite discreet about sex in the past," she says, laughing heartily, "But in this one, I couldn't stop writing the sex scenes. There was one point where I had to stop myself. It wasn't moving the plot forward, but I was having so much fun with it. I could have written a whole book of sex scenes, but I didn't want it to be Fifty Shades of Grey."

The Dubliner began with short stories in her late-20s, when she was living and working in London. She didn't start writing properly, however, until she faced up to her alcoholism and after she went through rehab.

Her first best-seller was Watermelon, about a woman whose husband leaves her after she gives birth to their baby, and her third, Rachel's Holiday, was a characteristically funny but moving account of a young woman in rehab.

Significantly, Marian did not turn to drink during the worst of her depression. "I had the feeling somewhere in me that I would be restored to myself some day," she explains, "and when I was restored, I wanted to find myself sober. I didn't want to find myself on another destructive route."

One massive lesson Marian has learned from these recent years is that it's a mistake to write off the bad bits of life as a mistake or a distraction from the main event of being happy.

"That's another orthodoxy I don't hold with," she says. "You don't get over everything. And that's okay, too. It's my journey and they're my feelings and they are legitimate. You can't polish off these things when they're over and put them away and say, 'That's grand now; let's get back to our perfect life.'"

This altered Marian Keyes is grateful, too, to find many things unaltered. Tony, her family, the people who love her, are still here, along with her sobriety and her writing. The cake-baking that got her through the early years of the depression is gone, however, replaced by her latest hobby.

"So it's chalk-painting now," she laughs, self-deprecatingly, explaining how she's "haunting' second-hand furniture shops for pieces to bring home and transform with brightly coloured paint, before passing them on.

Marian laughs heartily and happily at herself and where she is at in her life and you forget that there is any darkness behind the laughter, though she herself does not, maybe cannot, or cannot yet. "It's like a door is opened that you never knew was there," Marian explains of how depression took her by surprise and took her over.

"And once it has been opened, you cannot forget that it's there; the possiblity of awfulness has been opened and it's then there, forever, and you know that it can happen again. Once you know you're capable of that state of mind, you know you can return to it, so I'm grateful not to be in it.

"The shadow is always there. Before, I didn't know. Now, I know."

But that's not a bad thing, Marian is keen to explain. That is life and, more than anything else, Marian is grateful to have her life and to have clung to it. "With every one of us, we walk forward carrying our wounds," Marian says.

"But I'm okay. I'm different, but maybe I would have been different anyway. There's no way of knowing."